Quantum quest leads to super-efficient lights

A light that lasts 20 times longer than a conventional bulb and is 75% more energy efficient has been developed by researchers.

The light is based on organic light emitting diodes (OLEDs) that use a novel combination of photon-emitting compounds to convert more energy into light than existing versions. It was created by Stephen Forrest from Princeton University in New Jersey, and colleagues at the University of Southern California, both in the US.

OLEDs can easily be printed onto different surfaces and could therefore be used to illuminate buildings and homes in completely new and more energy efficient ways, experts say.

John de Mello, who works on OLEDs at Imperial College London, UK, and was not involved with the work, is impressed by the new work. “The beauty of this approach is that it harvests the energy in a near-optimal manner,” he says.

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De Mello adds that the OLEDs could make new methods of lighting possible. “The industrial interest in these materials stems from their ease of processing,” he says. “They can be deposited over large areas at relatively low cost.”

Red, green and blue

OLEDs produce light using a layer of semi-conducting material made from an organic compound. Passing a current through this layer raises the energy level of electrons inside it and as these electrons return to their previous energy they release photons.

The components can be made to produce different colours of light by adding “dopant” materials to change the wavelengths of emitted photons. Dopants that produce the right mix of red, green and blue light can be combined to fine-tune a material to a particular colour.

Phosphorescent molecules are normally used as the dopant material in OLEDs, as they release photons a steady rate and can be easily fine-tuned. But the molecules only convert a limited number of photons and some phosphorescent materials also tend to wear out rapidly.

Forrest and colleagues developed OLEDs that avoid both these problems, harnessing all the photons up for grabs, and that use only phosphorescent compounds that are long-lasting.

Singlets and triplets

They determined that OLEDs produce photons in one of two quantum spin states – referred to “triplets” and “singlets” – and that the phosphorescent material used in most OLEDs can only transmit triplets, which comprise 75% of all photons.

So Forrest’s team doped their OLEDs with fluorescent molecules as well as phosphorescent ones. The fluorescent molecules convert photons through a slightly different process, and can transmit singlets efficiently. As the fluorescent material produces photons at a blue wavelength, the researchers re-tuned the rest of the dopant material to produce the necessary wavelengths for white light. “Using this combination of fluorescence and phosphorescence means there’s no waste,” Forrest says.

The OLEDs developed by Forrest’s team have a longer lifespan than existing ones, because they use only longer lasting phosphorescent compounds “This first model has a lifetime of around 20,000 hours,” Forrest says.

By contrast, regular light bulbs – which supply about 40% of lighting in the US – normally only last for around 1000 hours, while compact fluorescent lights can last for around 10,000 hours. Lighting based on the new OLED should also be more efficient than conventional bulbs – they emit about 25 lumens per watt of energy, compared to about 15 for a normal bulb.